Highly Relativistic Electron Flux Enhancement During the Weak Geomagnetic Storm of April–May 2017

Abstract

Key Points:• Long-lasting multi-MeV electron enhancement during a period of a relatively weak geomagnetic storm not recorded in GEO. • Electron seed population was accelerated to relativistic energies by the enhanced chorus waves. • Relativistic electrons were further accelerated up to 10 MeV by inward diffusion Corresponding author: Ch. Katsavrias, ckatsavrias@phys.uoa.grWe report observations of energetic electron flux and Phase Space Density (PSD) to show that a relatively weak magnetic storm with Sym−H min ≈ −50nT, resulted in a relativistic and ultra-relativistic electron enhancement of two orders of magnitude similar to the St. Patrick's event of 2015, an extreme storm with Sym − H min ≈ −235nT. This enhancement appeared at energies up to ≈ 10 MeV, lasted for at least 24 days and was not recorded in geosynchronous orbit where most space weather alert data are collected. By combined analysis of PSD radial profiles and Fokker-Planck simulation, we show that the enhancement of relativistic and ultra-relativistic electrons is caused by different mechanisms: first, chorus waves during the intense substorm injections of April 21-25 accelerate the seed electron population to relativistic energies and redistribute them while inward diffusion driven by Pc5 ULF waves further accelerates them to ultra-relativistic energies.

Local heating of radiation belt electrons to ultra-relativistic energies

“…3 , highlighting enhancements. The multistep formation method for >3 MeV electrons, suggested by several authors 16 , 19 , would appear as the profile shown in Fig. 3 d. However, over a time-span of several days, the profiles for both storms shown in Fig.…”

“…3, highlighting enhancements. The multistep formation method for >3 MeV electrons, suggested by several authors 16,19 , would appear as the profile shown in Fig. 3d.…”

“…Focus has primarily been on lower values of μ (<6000 MeV G −1 ), leaving the dynamics of high energy particles largely unexplored. Here, we study the October 2012 period, where 7.7 MeV electron flux values, greater than the background threshold [16], are observed during multiple days, and calculate the phase space density values relating to several different μ, contrasting how electrons are enhanced over a broad range of energies. Figure 3 is a schematic illustration showing the evolving L* coverage of phase space density values within a factor of five of the maximum phase space density, for various acceleration mechanisms.…”

“…In the radiation belt region, local acceleration introduces radial gradients in phase space density and so is always accompanied by both outwards and inwards radial diffusion. Locally heating electrons to~7 MeV provides a very high energy "source population" for inwards radial diffusion and could therefore help explain the occurrence of~10 MeV electrons in April-May 2017 16 .…”

“…A multistep acceleration process has recently been suggested 13 – 15 , with particles first accelerated to relativistic energies locally by chorus waves and the newly formed population then further energized by inward radial diffusion 16 . However, radial profiles of phase space density relating to >3 MeV electrons have not been extensively studied and it remains unclear if relativistic electrons are accelerated to >1 MeV and then transported inwards, conserving the first and second invariants, or whether they can be locally accelerated directly to ultra-relativistic energies.…”